(b) $y_{2} = a_{2}\cos\left( \omega t - \frac{2\pi x}{\lambda} + \phi \right)$
$= a_{2}\sin\left\lbrack \frac{\pi}{2} + \left( \omega t - \frac{2\pi x}{\lambda} + \phi \right) \right\rbrack$
Compare it with $y_{1} = a_{1}sin\left( \omega t - \frac{2\pi x}{\lambda} \right)$
Phase difference$= \left( \frac{\pi}{2} + \phi \right)$
$\therefore\ $Path difference $= \frac{\lambda}{2\pi}\left( \frac{\pi}{2} + \phi \right)$